This invention relates, in general, to an improved process for the formation of self-supporting semiconductor layers, and, more specifically, to an improvement in the process which allows the shear separation of a semiconductor layer from a deposition substrate.
There is a need in the semiconductor industry and especially in that portion of the semiconductor industry concerned with the fabrication of photovoltaic devices for high quality, low cost semiconductor substrates. While the conventional technique of pulling a single crystal ingot, sawing it into wafers, and lapping and polishing the wafers to the desired thickness and surface finish is acceptable in the fabrication of many devices and integrated circuits, it is too costly for the large scale production of large area photovoltaic devices.
There has therefore been an effort to develop alternate sources of semiconductor material and especially for the production of thin sheets of silicon material in large area sheets or ribbons. One such technique is the ribbon-to-ribbon (RTR) process by which a polycrystalline ribbon or sheet is locally melted, for example through localized laser heating, to convert the polycrystalline material to a substantially monocrystalline or large grain polycrystalline ribbon or sheet. Producing the material in such thin sheets eliminates the need for any costly sawing, lapping, and polishing steps to achieve the necessary thin sheet semiconductor substrate.
To be successful, however, the RTR process requires a source of polycrystalline semiconductor material in sheet or ribbon form. There are, in turn, a number of techniques which have been proposed to produce such thin sheets of polycrystalline semiconductor material. One such technique, for example, is the chemical vapor deposition of polycrystalline silicon on a thermal expansion shear separation (TESS) substrate. Polycrystalline silicon is deposited at an elevated temperature and, upon cooling, the differential thermal expansion between silicon and the deposition substrate causes separation of a thin self-supporting polycrystalline sheet from the underlying substrate.
To be successful and practical, the substrates must be inexpensive or reuseable. The substrate must be refractory, contribute little unwanted dopant at the deposition temperature, and must have an expansion coefficient different from that of the semiconductor material being deposited. A further important requirement is that the semiconductor material not adhere to the substrate. Certain materials, for example sheet graphite and silicon dioxide, have expansion coefficients considerably different from silicon, but deposited silicon adheres to these substrate materials. The materials are therefore not usable as conventional TESS substrates. Upon cooling from the deposition temperature, the differential expansion coefficient causes shattering of the deposited silicon layer, not the separation in a unitary thin sheet.
Molybdenum is one material which meets most of the above enumerated criteria for a TESS substrate for the deposition of silicon. Molybdenum is refractory, contributes little doping to the deposited silicon, has a different thermal expansion coefficient than silicon, and does not adhere to the deposited silicon layer. Molybdenum is, however, a fairly expensive substrate material. The expense of the substrate is compounded by the fact that molybdenum substrates have a limited lifetime. As silicon is deposited on the molybdenum, a layer of molybdenum silicide forms at the substrate surface. In fact, this silicide layer participates in the shear separation mechanism. After a number of deposition cycles the amount of silicide builds up, a considerable amount of the molybdenum is consumed, and the substrate becomes brittle and unusable for subsequent depositions. The high cost of the molybdenum substrate transfers to the silicon being formed and makes the cost of the silicon excessively high.
Accordingly, in view of the need for low cost, self-supporting polycrystalline semiconductor sheets and ribbons and in view of the deficiencies in existing processes for obtaining such sheets and ribbons, there existed a need for a new and improved technique for producing such material.
It is therefore an object of this invention to provide an improved and economical process for producinng self-supporting sheets of semiconductor material.
It is a further object of this invention to provide an improved technique for the thermal expansion shear separation of a semiconductor material from a deposition substrate.
It is a still further object of this invention to provide an economical thermal expansion shear separation substrate.